Clad metal tubing, that is, a tube comprised of a tubular core or wire material internally or externally circumscribed by a dissimilar sleeve material, has been commercially available for many years. Offering a large array of differing physical and chemical characteristics, a composite structure may be fabricated to suit particular needs. The strength of the core material matched with selected properties of the cladding affords the engineer custom designed options that are superior to single material designs.
Various methods have been developed to coat metal surfaces. However, limitations exist with each presently known method. To illustrate, weld overlaying is commonly used to coat internal surfaces of articles of manufacture but such cladding requires essentially flat or cylindrical surfaces having little detail. There are also size limitations for such internal linings that are related to access by welding equipment. Similar limitations apply to the related processes, flame and plasma spraying which, although useful for internal cladding, provide coatings that may not be dense enough or thick enough for many applications. Welding and spraying methods can not be readily used to apply coatings of reactive metals such as titanium.
Although generally used on flat plate, explosive bonding and braze bonding are other methods that may be used for internal cladding. However, these processes are of limited use since they require precise mating of part and cladding.
Hot isostatic pressing is generally considered useful for forming powdered metal articles and is also useful for external metal cladding. It is conceivable that this process could also be used for the internal cladding of metal articles; however, the equipment used with this process is extremely sophisticated and requires considerable capital investment.
Composite tubing can be prepared by simultaneous extrusion of a powdered metal and a solid shell. This method is applicable to many materials. Unfortunately, high production costs translate into high selling prices and as a consequence limit the usage of these materials.
Turning now more particularly to extruding methods, a major difficulty is caused by the need for an intermediate can. The can must be first fabricated, positioned in place, processed and ultimately removed; each step increasing the cost of the technique. Moreover, as the diameter of the ultimate product becomes smaller, i.e. welding electrode, the use of a can becomes less desirable.
Representative examples of the prior powder art include U.S. Pat. No. 2,390,452 which teaches a method for welding dissimilar materials and U.S. Pat. Nos. 3,652,235 and 3,753,704 (a continuation-in-part thereof) which disclose an isostatic method of fabricating a clad product. U.S. Pat. No. 4,016,008 utilizes an internal metal liner or can which is generally removed. U.S. Pat. No. 4,065,306 utilizes an expandable bladder.
The instant invention is directed toward the expeditious production of tubular or rod goods of relatively lower cost and compatible quality when compared to existing methods.